1. Field of the Invention
The present invention relates to a ring resonator comprising a planar conducting ring arranged on a substrate beside a planar conductor of a microwave circuit. This type of ring resonator is also used in a microwave circuit in order to, e.g., tune an oscillator to a desired resonance frequency or in order to filter out a predetermined small frequency band from a larger frequency range.
2. Prior Art
A ring resonator whose conducting ring is dimensioned in regard to its width and its diameter so that it is driven at its fundamental frequency is described in the article, "Varactor Tuned Ring Resonator Microwave Oscillator", Electronics Letters, Vol. 32, No. 1, 1996, pp. 46 to 48, by Shih-Lin Lu and A. M. Ferendeci. Two closely neighboring modes occur at this fundamental frequency, one of which must be suppressed. That happens by means of a slot provided in the conducting ring. The ring resonator has a higher radiation of electromagnetic radiation because of this slot and thus the Q-factor of the resonator is disadvantageously reduced. The resonance frequency depends strongly on the spacing of the ring resonator from the cover of the housing in which the microwave circuit with the ring resonator is mounted because of the high radiation from the resonator. The resonance frequency is subjected to undesirable changes because of variations of the spacing of the housing cover from the ring resonator--originating from the thermal expansion of the housing or from mechanical vibrations.